The Hollywood celebs cut weight for roles ranging from an AIDS patient to a destitute prostitute through a simple (but extreme) method: total or near-fasting.

“It takes a while for your body to understand that it has to feed off itself, and that you’re not going to give it something else from the outside,” McConaughey toldABC. “I should not look healthy by the time I’m doing this.”

Within the first 24-48 hours of calorie deprivation, the body depletes its glycogen stores from muscles and the liver — basically, that’s the carbohydrate stores that provide quick energy, Dubost said. Then it starts breaking down protein within muscles and fat to produce energy. During that process, the body’s metabolism slows down in order to conserve energy, she said. Meanwhile, hormonal reactions will fluctuate.

If you’re also forgoing water, things go downhill even more quickly, Dubost said. Dehydration can lead to low blood pressure and cardiac arrest.

“So for the first day of shooting I didn’t drink water or any liquids for 36 hours, which a trainer at the time said is a bodybuilder trick which can give you sunken cheeks and sunken eyes,” Jackman toldThe Sun. “So I went for it — and slightly regretted it 20 hours in. When I see the first shot I am really glad I did it. But I had massive headaches, was slightly dizzy and very, very grumpy.”

What Jackman was experiencing, Dubost said, was symptomatic of dehydration — which, if ignored, can carry ramifications such as brain damage, seizure, and, in extreme cases, death.

“This isn’t something that should be taken lightly,” Dubost said. “Unfortunately, a lot of people look at what celebrities do and think they can take it on as well, but that’s really not the case.”

While Dubost sees no benefits in complete fasts, others think that a milder form of fasting could help certain populations. Intermittent fasting, proponents say, produces a milder stress on the body similar to exercise.

“Exercise is an energetic stress, where you have a marked increase in energy expenditure,” said Mark Mattson, chief of the laboratory of neurosciences at the National Institute on Aging. An approach like intermittent fasting may work in a similar way, he says, “by putting an energetic stress on the body, but in kind of the opposite way, through reduced energy availability.”

And the adaptive responses to that mild stress of not having food? In animal studies, Mattson and other researchers have found positive changes in nerve cells. Intermittent fasting can protect neurons from becoming damaged by oxidative stress, for example, and better regulate glucose.

And in human research, one study showed women at risk for breast cancer both burned more fat and showed greater insulin sensitivity on a diet with two extremely low-calorie days than those who consumed a lowered, but consistent, number calories per day.

In fact, Mattson says there’s evidence from some animal studies and emerging evidence in humans that the risk of some major diseases that there is currently no treatment for, such as Alzheimer’s, might be reduced by certain types of diet and exercise.